file.tell() -- obscure IOError

[Uwe Mayer]

hi,

i have an object which conforms to the file protocoll. while calling the 
self.file.tell()   method, i get the following error message:

>>> s1 == s2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "riff.py", line 153, in __cmp__
    data1 = self.read(self.value.step)
  File "riff.py", line 205, in read
    maxsize = len(self) -self.tell()
  File "riff.py", line 234, in tell
    return self.file.tell() -self.value.start
IOError: (0, 'Error')

when does tell() raise an IOError?

Thanks for any help!
Yours
Uwe